1. Field of the Invention
The present invention relates to an optical disc including a number of data storage layers that are stacked one upon the other, and to an optical disc drive for reading and/or writing data from/on such an optical disc effectively.
2. Description of the Related Art
The density of data that can be stored on a data storage layer of a given optical disc (which will be referred to herein as a “storage density”) is inversely proportional to the area of a beam spot to be formed by a laser beam on the data storage layer in reading or writing data from/on that data storage layer. In this case, the beam spot area is proportional to the square of the spot size of the laser beam. The laser beam spot size is, in turn, proportional to the wavelength of the laser beam and inversely proportional to the numerical aperture NA of an objective lens, which is built in the optical head of an optical disc drive. It should be noted that the “optical disc drive” herein refers to not just read/write drives, players and camcorders for various optical discs but also any other general electronic device as well, as long as the device uses an optical disc as its storage medium.
In a CD (compact disc), which is probably the most known type of optical disc, its compatible objective lens has a numerical aperture NA of 0.45, a laser wavelength of 780 nm and a storage capacity of 650 MB. As for a DVD (digital versatile disc) on the other hand, its compatible objective lens has a numerical aperture NA of 0.6, a laser wavelength of 680 nm and a storage capacity of 4.7 GB. And in a BD (Blu-ray Disc) that has been put on the market just recently, its compatible objective lens has a numerical aperture NA of 0.85, a laser wavelength of 405 nm and as big a storage capacity as 25 GB.
In this manner, the optical discs have increased their storage capacities by leaps and bounds by increasing their numerical apertures NA and shortening their laser wavelengths.
However, it is currently very hard to further increase the numerical aperture NA of the objective lens and make the wavelength of the laser beam even shorter. The reasons are as follows.
Firstly, the numerical aperture NA of an objective lens is represented by the sin function of an angle, which is defined by the focal point and effective radius of the lens. Accordingly, the greater the numerical aperture NA, the narrower the gap between the optical head and the optical disc. For example, if the numerical aperture NA is 0.85, this gap is about 0.1 mm. As the head-to-disc gap narrows in this manner, it becomes increasingly more difficult to control the focal point of the objective lens just as intended.
It is also extremely difficult to further shorten the wavelength of the laser beam because that requires development of a brand-new type of semiconductor devices. As of today, it still looks like a long way off for any semiconductor laser diode to achieve as short an oscillation wavelength as 405 nm, or even less. Somebody suggested using a nonlinear optical element such as a second harmonic generator (SHG) to convert the laser beam radiated from a semiconductor laser diode into a radiation with an even shorter wavelength. Unfortunately, though, this technique is still far from being a realistic idea.
While it is extremely difficult to further increase the numerical aperture NA of the objective lens and make the laser beam wavelength even shorter under circumstances such as these, the development of optical discs with even greater storage capacities is awaited. This is because nowadays the users' demand on storage capacity has gone far beyond the maximum storage capacities of currently available optical discs. For instance, if a 25 GB BD is used to store an HDTV (high definition TV) broadcast program thereon, then just two hours is the limit for a single BD. Meanwhile, as the broadband Internet access and permanent connection has become more and more common service recently for general household users, the quantity of digital data to be provided for them is on a steep rise. For example, if a user continues to receive contents for 24 hours at an average transfer rate of 3 Mbps, then the total quantity of data received will amount to 32 GB. To store such a huge quantity of data, at least two 25 GB BDs are needed.
To overcome these problems, multilayer optical discs are now under research and development. For example, as for a BD, a “two-layered disc” including two data storage layers on a single base member has already been standardized and has a storage capacity of 50 GB. Thus, many people believe a multilayer optical disc is a way to go to increase the storage capacity of an optical disc tremendously.
An optical disc including a number of data storage layers that are stacked on a single base member is disclosed in Japanese Laid-Open Publications No. 2000-235732 and No. 11-195243, for example. According to the techniques disclosed in these documents, to increase the storage capacity of an optical disc, it is indispensable to increase the NA. More specifically, the condition NA/λ≧1.20 must be satisfied.
However, what turned out to be effective in increasing the storage capacity of a single-layer optical disc is not always applicable as it is to increasing the overall storage capacity of a multilayer optical disc. For example, even if the numerical aperture NA of the objective lens is increased, the overall storage capacity does not increase proportionally to the square of the numerical aperture NA. This is because if the numerical aperture NA is increased for a multilayer optical disc, the wave aberrations (such as coma aberration and astigmatism) increase, thus making it more important to define a desired angle between the optical axis of the incoming light and a normal to the data storage layers.
Supposing the tilt angle is constant, the wave aberration (i.e., coma aberration or astigmatism) increases proportionally to the third power of the numerical aperture NA or more. The tilt angle is constantly variable around zero. Accordingly, supposing the tilt angle has a fixed variation range, the decrease in beam spot size due to the increase in numerical aperture NA is canceled by the increase in beam spot size due to the increase in wave aberration. Consequently, unlike the storage capacity of a single-layer optical disc, the overall storage capacity of the respective data storage layers included in a multilayer optical disc does not increase proportionally to the square of the numerical aperture NA.
For that reason, to increase the overall storage capacity (i.e., the sum of the storage capacities) of the respective data storage layers, not just the storage capacity of each single data storage layer but also the number of data storage layers included in one optical disc should be increased. And to increase the number of data storage layers included in a given optical disc, the distance from the light incident side of the optical disc to the deepest one of the data storage layers thereof, which is located most distant from the light incident side (i.e., the storage layer stack thickness) needs to be increased and the interval between adjacent data storage layers needs to be decreased.
However, if the storage layer stack thickness is increased, then the tilt angle will have more and more significant effects. For example, suppose a wave aberration is 34.5 m λ when the storage layer stack thickness is 0.1 mm and the tilt angle is 0.31 degrees. In that case, if the storage layer stack thickness is increased to 0.2 mm with the tilt angle maintained at 0.31 degrees, then the wave aberration will increase to 69 m λ. Stated otherwise, to maintain the wave aberration at 34.5 m λ at a storage layer stack thickness of 0.2 mm, the tilt angle needs to be decreased to 0.155 degrees.
As can be seen, to keep the wave aberration constant, the tilt angle needs to be decreased as the storage layer stack thickness increases. The tilt angle depends on not only the tilt and roughness of the optical disc itself but also the positional relationship between the optical disc and the optical pickup as well. If the tilt angle needs to be decreased, an optical disc drive should include a tilt control mechanism for controlling the tilt angle to zero degrees while an optical disc is subjected to a read or write operation. However, even if such a tilt control mechanism is provided, the tilt angle is still not always equal to zero degrees. This is why if the tilt angle tolerance should be decreased, then the tilt control mechanism needs to perform its control operation even more precisely.
On the other hand, if the storage layer stack thickness is decreased, then the layer-to-layer interval needs to be decreased, too. In such a situation, the crosstalk noise should increase. The crosstalk noise diminishes as the quantity of data covered by a defocused beam on an upper or lower adjacent data storage layer increases.
This is because if the quantity of data, covered by a defocused beam on the adjacent data storage layer, increases, then the ratio of the total area of data pits to that of non-data pits and the quantity of light reflected from the adjacent data storage layer will be closer to their respective constant values.
Conversely, if the numerical aperture NA is increased with the layer-to-layer interval kept constant, then the crosstalk noise decreases. This is because when the numerical aperture NA increases, the defocused beam on an adjacent data storage layer increases not only its size but also the quantity of data included there as well. Accordingly, by increasing the numerical aperture NA, the layer-to-layer interval can be narrowed due to the decrease in crosstalk noise. Furthermore, when the numerical aperture NA is increased, the beam spot size decreases and the storage capacity of each data storage layer increases as described above.
As is clear from the foregoing description, the overall storage capacity (i.e., the sum of the respective storage capacities) of data storage layers in a multilayer optical disc cannot be increased sufficiently just by increasing the numerical aperture NA. Also, if the storage layer stack thickness is decreased with the numerical aperture NA increased, then the increase in aberration can be reduced but the number of data storage layers included needs to be decreased. As a result, the overall storage capacity may not increase.
Thus, to increase the overall storage capacity of a multilayer optical disc, not just the numerical aperture NA and storage layer stack thickness but also other parameters such as a tilt angle tolerance and a layer-to-layer thickness need to be taken into account. In the prior art, however, there are no guidelines on how these parameters should be defined to increase the overall storage capacity effectively. In view of these considerations, if a normal multilayer optical disc is going to include five or more data storage layers in the near future, then the conventional technology will be quite inept at determining the number of data storage layers, numerical aperture NA and other parameters properly.